We previously generated cell hybrids between a derivative of the E6-containing HeLa cell line and a p53 null peripheral neuroepithelioma (PNET) cell line. Although p53 protein from the hybrids was genotypically wild type, it did not demonstrate wild-type behavior. Therefore, in the present study, we introduced wild-type p53 into the PNET parent to investigate whether p53 retained wild-type function within this cell line. Although the p53 null PNET parent lacked detectable p21 protein, introduction of wild-type p53 resulted in a detectable expression of p21 protein in all clones tested, suggestive of wild-type p53 function. In addition, p53 expression was necessary for induction of p21 in response to irradiation, and, furthermore, we show this induction to occur at the transcriptional level. Although introduction of wild-type p53 seems to be responsible for p21 induction, the overall protein levels of p53 were not induced. The involvement of p53 in up-regulating p21 is further substantiated by the observation that p21 up-regulation was dependent on the introduction of the wild-type protein. Our results suggest that wild-type p53 is capable of up-regulating p21 in response to DNA damage in the absence of p53 induction.